Evaluation of the Relationship between Blocking Patterns and Duration of Spring Frost Waves: The Case of Iran

In order to evaluate the effects of blocking patterns on the duration of frost waves across Iran, the minimum daily temperatures of 60 weather stations were collected from 20 March 1968 to 21 June 2014. The findings, which were obtained based on the distribution tables of reported temperatures and their frequency variation graphs, resulted in identifying 114 frost waves. We obtained the daily weather maps at 500 hPa from NCEP/NCAR during the peak days of durable frosts, and it was revealed that the formation of low pressures over 20°–70°E and the dominant southwest–northeast troughs at 500 hPa facilitated the influx of cold air from both eastern Europe and central Siberia toward Iran, leading to the average frost duration of 12.4 days. Furthermore, low-pressure centers appeared in a dipole pattern in northern Iran over Caspian Sea and created durable frosts with an average duration of 15.4 days. The effect of omega shaped pattern of the geopotential height on durable frosts demonstrated a sharp difference with two recent patterns leading to the frost occurrence with a 13.3-day duration averagely. Additionally, the slow speed of cold western currents in the rex type of blocking from 30° to 50°N caused durable frosts of 14.3 days. Also, we found a specific reduction in the frequency of blocking patterns, which has occurred in the past four decades. Mostly, the blocking and non-blocking patterns cause long- and short-duration frosts over Iran, respectively.